15_15_87 - DIELECTRIC STRENGTH OF INSULATING MATERIALS L I...

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Unformatted text preview: DIELECTRIC STRENGTH OF INSULATING MATERIALS L. I. Berger The loss of the dielectric properties by a sample of a gaseous, liquid, or solid insulator as a result of application to the sample of an electric field* greater than a certain critical magnitude is called dielectric breakdown The critical magnitude of electric field at which the breakdown of a material takes place is called the dielec- tric strength of the material (or breakdown voltage ) . The dielectric strength of a material depends on the specimen thickness (as a rule, thin films have greater dielectric strength than that of thicker samples of a material), the electrode shape**, the rate of the ap- plied voltage increase, the shape of the voltage vs . time curve, and the medium surrounding the sample, e .g ., air or other gas (or a liquid — for solid materials only) . Breakdown in Gases The current carriers in gases are free electrons and ions gener- ated by external radiation . The equilibrium concentration of these particles at normal pressure is about 10 3 cm –3 , and hence the elec- trical conductivity is very small, of the order of 10 –16 – 10 –15 S/cm . But in a strong electric field, these particles acquire kinetic energy along their free path, large enough to ionize the gas molecules . The new charged particles ionize more molecules; this avalanche- like process leads to formation between the electrodes of channels of conducting plasma (streamers), and the electrical resistance of the space between the electrodes decreases virtually to zero . Because the dielectric strength (breakdown voltage) of gases strongly depends on the electrode geometry and surface condition and the gas pressure, it is generally accepted to present the data for a particular gas as a fraction of the dielectric strength of either nitrogen or sulfur hexafluoride measured at the same conditions . In Table 1, the data are presented in comparison with the dielectric strength of nitrogen, which is considered equal to 1 .00 . For con- venience to the reader, a few average magnitudes of the dielectric strength of some gases are expressed in kilovolts per millimeter . The data in the table relate to the standard conditions, unless in- dicated otherwise . Breakdown in Liquids If a liquid is pure, the breakdown mechanism in it is similar to that in gases . If a liquid contains liquid impurities in the form of small drops with greater dielectric constant than that of the main liquid, the breakdown is the result of formation of ellipsoids from these drops by the electric field . In a strong enough electric field, these ellipsoids merge and form a high-conductivity channel be- tween the electrodes . The current increases the temperature in the channel, liquid boils, and the current along the steam canal leads to breakdown . Formation of a conductive channel (bridge) between the electrodes is observed also in liquids with solid impurities . If a liquid contains gas impurities in the form of small bubbles, break-...
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15_15_87 - DIELECTRIC STRENGTH OF INSULATING MATERIALS L I...

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